Conventionally, a cold-cathode tube was, for example, used as a light source for a lighting system or a liquid crystal display backlight and the like. But in recent years, a pseudo-white light source, which is a combination of a light source emitting blue light and a material absorbing blue light and emitting yellow light, has been developed as an alternative light source. In this pseudo-white light source, an InGaN base light-emitting diode is used as a light source emitting blue light and cerium-activated yttrium aluminate is used as a material emitting yellow light, for example.
However, there are intrinsic insufficiency of green light and red light components in the spectrum of the light emitted from the pseudo-white light source, thereby the pseudo-white light source is low in color rendering, as well as in color reproduction. In order to solve this problem, the way of enhancing the color rendering and the color reproduction is proposed by means of adjusting the component of yttrium aluminate (that is the material emitting yellow light) for the purpose of improving it to emit yellow-green light and, at the same time, adding a material that absorb blue light and emit red light to the yttrium aluminate for the purpose of make up the shortage of the red light component of the pseudo-white light source.
However, materials emitting red light often absorb not only blue light but also lights with wavelength components longer than that of blue light and shorter than red light, or green to yellow light. The examples of such materials are alkaline-earth metals sulfide activated by europium, nitride of alkaline-earth metals and silicon activated by europium, oxynitride of alkaline-earth metals and silicon activated by europium or the like. These materials generally absorbs 400 nm to 580 nm wavelength light efficiently, and emit orange to red light, of which wavelength peak is located at about 580 nm to 680 nm.
Materials emitting orange to red light, typified by the above-mentioned ones, absorb the lights with shorter wavelengths than those of themselves, or green to yellow lights. Therefore, in case materials emitting orange to red light and materials emitting green to yellow light are used together, the orange to red light material absorbs a part of the light from green to yellow light material, thereby the luminous flux of the light emitting device gets remarkably lowered.
Several solutions are being tested at present, in which the reduction in luminous flux, occurred from the absorption of shorter-wavelength light into the materials emitting longer-wavelength light, could be prevented. For example, Patent Document 1 discloses that, in a light emitting device comprising two sorts of materials (here called, “material A” and “material B”) that absorb the light from a light source and emit lights with different wavelengths respectively, when the material A (which corresponds to a material emitting orange to red light) absorb a part of the light emitted from the material B (which corresponds to a material emitting green to yellow light), the improvement in color rendering and the prevention of reduction in luminous flux can be achieved by locating the material A closer to the light source than the material B is.
[Patent Document 1] Japanese Patent Laid-Open Publication (Kokai) No. 2004-71726